Search system

ABSTRACT

2. A search system for detecting moving targets, in chaff, and including a transmitter for reflecting bursts of randomly phase coded, electromagnetic energy from a target, and a receiver, said receiver comprising A DELAY LINE HAVING A PLURALITY OF OUTPUT TAPS THEREON, MEANS FOR APPLYING SAID REFLECTED BURSTS TO SAID DELAY LINE, A PLURALITY OF PHASE SHIFTERS WHICH ARE PRESET IN ACCORDANCE WITH THE PHASE CODING OF SAID TRANSMITTER, EACH PHASE SHIFTING MEANS BEING CONNECTED TO A DIFFERENT OUTPUT TAP OF SAID DELAY LINE, SAID PLURALITY OF PHASE SHIFTERS REMOVING SAID PHASE CODING FROM SAID BURSTS AND PROVIDING A PURE Doppler signal output, and DISPLAY MEANS OPERABLY CONNECTED TO RECEIVE THE OUTPUT FROM EACH OF SAID PHASE SHIFTING MEANS FOR INDICATING THE Doppler shift introduced by the target to each pulse of said burst.

Unite States Patent [is] 3,706,993

Kuck [451 Dec. 19,1972

[54] SEARCH SYSTEM EXEMPLARY CLAIM Inventor; J Kuck, Silver Spring, 2. Asearch system for detecting moving targets, in

[73] Assigneez The United states 0! America as chaff, and including atransmitter for reflecting bursts represented by the Secretary of the ofrandomly phase coded, electromagnetic energy Navy from a target, and areceiver, said receiver comprlsing a delay line having a plurality ofoutput taps [22] Filed: March 9, 1964 thereon,

[21] APPL No; 350,951 means for applying said reflected bursts to saiddelay line, a plurality of phase shifters which are preset in ac- [52]US. Cl. ..343/7.7, 343/8, 343/9, rdance with the phase coding of saidtrans- 343/17.l R mi en 511 int. Cl .0015 9/42 each Phase shifting meansbeing connected to a [58] Field of Search ..343/5, 7.7, 8,9, 11, 13, 14,few!" Output tap of Said delay line Said plurality 343/17 1 R, of phaseshifters removing said phase coding from said bursts and providing apure Doppler signal [56] References Cited 9 and display means operablyconnected to receive the UNITED STATES PATENTS output from each of saidphase shifting means for indicating the Doppler shift introduced by thetar- 3,217,324 ll/l965 Adamsbaum et al. ..343/l7.l R get to each pulseof Said burst 3,302,162 1/1967 Rowlands ..343/7.7 3,526,894 9/1970Guilhcm et a]. ..343/9 5 Claims, 1 Drawing Figure Primary Examiner-T. H.Tubbesing Attorney-Q. Baxter Warner and Claude Funkhouser summsa SUMME vT SUMMER THRESHOLl) DETECTION /6 MEMORY AND Eso-ou'r TARGET, RANGE ANDVELOCITY SEARCH SYSTEM The present invention relates in general to radarsystems and more particularly to a novel moving target indicating searchsystem.

A search system has been proposed for detecting moving targets in chaffwhich utilizes transmission on a number of fixed frequencies.Intercomparison between different transmitted frequencies havingslightly dif ferent pulse repetition frequencies was used to eliminaterange ambiguities. Storage tubes and Doppler filter banks were used inthe data processor.

The present invention provides a system in which the type oftransmission is similar to the previously proposed system, however, onlya single frequency need be transmitted. Range ambiguities arecompensated for by randomly phase coding the transmitted pulses anddecoding them on reception by a delay line matched filter technique.Although the system requires transmission of only a single frequency,frequency diversity may be obtained by duplication of equipment ondifferent frequencies and adding outputs after detection. The matchedfilter principle utilized is similar to the well-known pulse compressiontechnique.

An object of the present invention is to provide a radar system whichutilizes a single pulse repetition frequency to eliminate rangeambiguities.

Another object of the present invention is to provide a radar systemwhich utilizes random phase coding of a single transmitting frequency toeliminate range ambiguities.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing, wherein:

The FIGURE is a block diagram of the system constituting the presentinvention.

The system operates on the basis of 100 transmitted phase coded pulsesper burst. The Doppler signals are picked up at a receiver 1 and appliedto a 100 tap delay line 2 which has a delay time comparable to the timeof transmission of each 100 pulse burst. The output of each tap in delayline 2 is then applied to a phase shifter 3 which is set according tothe phase coding of the transmitted signals. For convenience only threeof the phase shifters have been shown. The result is that the outputs ofthe phase shifters 3 are pure Doppler signals, all occurring in phaseexcept for the Doppler phase shift introduced by the target. The delayline 1 has compensated for the time difference between the pulses intransmission and the phase shifters 3 have a compensated for the randomphase coding in the transmitter.

In view of the foregoing discussion, it should be evident that althoughthe various pulses comprising each transmitted burst have been referredto as being randomly phase coded, this phrase is not intended to in anyway mean that the phase relationship between one transmitted pulse andanother is unknown. Instead, this phrase merely means that the selectedphase difference between successive pulses does not follow a uniformpattern; i.e., the phasing from pulse to pulse is varied according to aselected, yet random, pattern so that the coding of the radar signalwill not be readily discernible by an enemy, for example. Moreover,since the phase coding of the transmitted pulses is known, the phaseshifters 3 at the output of the delay line 1 can then be preset, eithermanually or electronically, to provide the proper amount of phaseshifting to each received pulse which is necessary to remove the phasedifferences with which the radar signal is coded. As previouslymentioned, the illustrated receiving apparatus is thus capable ofreadily distinguishing and extracting the radar information from theso-called chaff.

In order to detect the Doppler phase shift introduced by the target, aplurality of groups of fixed phase shifters 4 are provided. Each groupof phase shifters 4 is set to cover one of the many Doppler frequencyintervals expected to be covered by the system. The output of each ofthe phase shifters 3 is applied to an appropriate one of the phaseshifters 4 in each Doppler frequency group. The outputs of all of thephase shifters 4 in a given group are then summed in a summer 5 and thenapplied to a detection, memory, and read-out system 6.

The theory of operation is as follows. The phase of the targetreflection of the last transmitted pulse in the group of transmittedpulses may be expressed as:

+4rrrlh 1 where (1: is the phase of the coherent transmitter sourcebefore the addition of random phase coding, d), is the random phasecoding that is added in the transmitter, r is the target range and h isthe transmitted wavelength. Since this is the last pulse in the group itwill be fed into the summers 5 undelayed.

The phase of any earlier pulse transmitted at a time t,, before the lastpulse will be:

(4 1r r/h) wt,,+ (4rrvtn/h) 2 where V equals the target velocity in aradial direction. It is desired to remove the phase differences betweenthe expressions in equations l and (2) so that all pulses will haveequal phase when summed. To accomplish this a delay line 2 is chosensuch that the nth pulse is delayed by t,,. Thus, the nth pulse willappear at this tap at the same time that the last pulse enters thesummer 5 from the undelayed point at the delay line input. In addition,the delay of t compensates for the term --wt, in equation (2) so thatthis term does not appear in the phase shift of the nth pulse at the nthtap.

From each tap of the delay line 2 the signal enters one of the phaseshifters 3 whose phase shift is set at (1), thereby eliminating it fromthe expression for the phase of the nth pulse. If the transmitted phasecoding is restricted to 0 and then the phase shifters 3 can be invertingamplifiers that can be switched in and out.

Upon emerging from a phase shifter 3 each signal line splits into 70channels and is carried to seventy different phase shifters 4 (only sixbeing shown), one con nected to each of 70 summers 5 (but three beingshown). Each summer will give a peak response in a different one of the70 different Doppler frequency intervals that are to be covered by thesystem. The function of the phase shifters 4 is to remove the (41rvtn/A)term. When the nth pulse from the nth delay line tap has passed throughthe phase shifter 4 connected to the last summer 5, its phase shift willbe:

l060ll 0823 When the target velocity V equals the center velocity V ofthe kth channel, the quantity (41rtn/A)( VV is zero. The pulses from allthe delay line taps are then in phase and add to give maximum responsein the output of the summer.

The outputs of the seventy summers carry simultaneously the informationon all targets in the seventy velocity intervals. These pulses may bedetected by the threshold detectors, and the range and velocityinformation may then be stored in the memory and read out sequentially,the detection, memory and read-out being represented by the block 6.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A search system for detecting a moving target in the presence ofchaff, comprising a receiver,

a delay line connected to the receiver,

a plurality of phase shifters connected to the delay line,

a second group of phase shifters connected to each of thefirst-mentioned phase shifters,

a summer connected to selected phase shifters of the second group,

a threshold detector connected to the summer,

a memory storage unit connected to the detector,

and

a read-out device connected to the memory storage unit.

2. A search system for detecting moving targets, in chaff, and includinga transmitter for reflecting bursts of randomly phase coded,electromagnetic energy from a target, and a receiver, said receivercomprising a delay line having a plurality of output taps thereon,

means for applying said reflected bursts to said delay line,

a plurality of phase shifters which are preset in accordance with thephase coding of said transmitter, I

each phase shifting means being connected to a different output tap ofsaid delay line, said plurality of phase shifters removing said phasecoding from said bursts and providing a pure Doppler signal output, and1 display means operably connected to receive the output from each ofsaid phase shifting means for indicating the Doppler. shift introducedby the target to each pulse of said burst.

3. The search system of claim 2, said receiver further including,

a plurality of summing circuits, the number of summing circuits beingequal to the number of Doppler frequency intervals over which the systemis operative,

means for connecting the output of each of said phase shifters to eachof said summing circuits, and

means for connecting the outputs of each summing circuit to said displaymeans, whereby the Doppler shift of said burst due to the presence of atarget may be detected over a range of Doppler frequency intervals. 4.The search system of claim 2, said receiver further including,

a threshold detector, said detector securing the outputs of said summingcircuits and selecting for display the output of maximum signalintensity.

1. A search system for detecting a moving target in the presence ofchaff, comprising a receiver, a delay line connected to the receiver, aplurality of phase shifters connected to the delay line, a second groupof phase shifters connected to each of the firstmentioned phaseshifters, a summer connected to selected phase shifters of the secondgroup, a threshold detector connected to the summer, a memory storageunit connected to the detector, and a read-out device connected to thememory storage unit.
 1. A search system for detecting a moving target inthe presence of chaff, comprising a receiver, a delay line connected tothe receiver, a plurality of phase shifters connected to the delay line,a second group of phase shifters connected to each of thefirst-mentioned phase shifters, a summer connected to selected phaseshifters of the second group, a threshold detector connected to thesummer, a memory storage unit connected to the detector, and a read-outdevice connected to the memory storage unit.
 3. The search system ofclaim 2, said receiver further including, a plurality of summingcircuits, the number of summing circuits being equal to the number ofDoppler frequency intervals over which the system is operative, meansfor connecting the output of each of said phase shifters to each of saidsumming circuits, and means for connecting the outputs of each summingcircuit to said display means, whereby the Doppler shift of said burstdue to the presence of a target may be detected over a range of Dopplerfrequency intervals.
 4. The search system of claim 2, said receiverfurther including, a plurality of summing circuits equal in number tothe number of Doppler frequency intervals within the operating range ofsaid system, and means connecting the outputs of said summing circuitsto said display means.
 5. The search system of claim 4 in which saiddisplay means includes, a threshold detector, said detector securing theoutputs of said summing circuits and selecting for display the output ofmaximum signal intensity.